1. Field of the Invention
This invention relates to a waveguide type optical branching device and, particularly, to a waveguide type optical branching device with an X-branching optical circuit for branching light at a specified branching ratio.
2. Description of the Related Art
Optical communication system access networks have as their indispensable constituent element an optical branching device (a 2×N coupler) for causing 1.25-1.65 μm wavelength incident light from two input-side optical fibers to branch into plural output-side optical fibers at a constant branching ratio regardless of wavelengths, polarizations, and input ports.
The 2×N coupler typically comprises a 2-input and 2-output optical branching device (a 2×2 coupler) with a constant branching ratio, and two 1-input and M-output optical branching devices (a 1×M coupler, M=N/2) respectively connected to the output ports of the 2×2 coupler.
Because the optical properties of the 2×2 coupler are generally poor in comparison to the optical properties of the 1×M coupler, the optical properties of the 2×N coupler are restricted mainly by the optical properties of the 2×2 coupler. Accordingly, in order to realize a high-performance and low-cost 2×N coupler, it is important to realize a 2×2 coupler with a constant branching ratio regardless of wavelengths, polarizations, and input ports.
Because the 2×2 coupler is one of the most basic constituent elements of the waveguide type optical branching device, if a 2×2 coupler is realized with a constant branching ratio regardless of the conditions such as wavelengths, etc., it is also very useful in applying it to optical communication components other than access networks, or optical waveguide components used in fields other than optical communications
As techniques for realizing a 2×2 coupler whose branching ratio does not depend on wavelengths, polarizations, and input ports, there are known a fusion coupler using a fused and drawn optical fiber, a Mach-Zehnder interferometer-waveguide type wavelength-independent coupler (MZI-WINC), a waveguide type optical branching device (a waveguide type coupler) using an asymmetrical X-branching optical circuit, and a waveguide type optical branching device using an adiabatic coupler type optical circuit.
As shown in FIG. 12, a waveguide type optical branching device 90 using an adiabatic coupler type optical circuit comprises two input ports 91a and 91b; two output ports 92a and 92b; and optical waveguides 93 and 94 for connecting the input ports 91a and 91b and the output ports 92a and 92b, respectively. The two optical waveguides 93 and 94 gradually approach each other to form a coupling portion 95 comprising the two parallel optical waveguides 93 and 94 close to each other. As shown in FIG. 13, the two optical waveguides 93 and 94 comprise a core 97 formed on a substrate 96 and with a rectangular cross section in the light propagation direction, and cladding 98 that covers the core 97. The G in the figure denotes the length of a gap between two cores 97 in the coupling portion 95.
A light signal input from one input port 91a of the waveguide type optical branching device 90 is mode-converted to be caused to branch at the optical power ratio of substantially 1 to 1 and output from the two output ports 92a and 92b (See JP-B-3225819, and Y. Shani et al., “Integrated Optic Adiabatic Devices on Silicon”, IEEE, J. Lightwave Technol., 1991, vol. 27, p.p. 556-566).
As shown in FIG. 14, a waveguide type optical branching device 100 using an asymmetrical X-branching optical circuit comprises, on a substrate not shown, two input ports 101a and 101b; two output ports 102a and 102b; and a coupling portion 107 for coupling, in an X shape, optical waveguides 103, 104, 105 and 106 from the input ports 101a and 101b and the output ports 102a and 102b respectively. The two optical waveguides 103 and 104 on the input port 101a and 101b side have the same core width, while the two optical waveguides 105 and 106 on the input port 102a and 102b side are formed so that one optical waveguide 105 has a large core width and the other optical waveguide 106 has a small core width.
In the waveguide type optical branching device 100 of FIG. 14, a light signal input from one input port 101a is caused to branch at the ratio of substantially 1 to 1 in the coupling portion 107 and output from the two output ports 102a and 102b (See M. Izutsu, A. Enokihara, T. Sueta, “Optical-waveguide hybrid coupler”, OPTICS LETTERS, 1982, Vol. 7, No. 11, p.p. 549-551).
However, there is the problem that the 2×2 coupler using the fusion coupler, or MZI-WINC tends to cause fabrication errors in the optical branching device to be fabricated, resulting from its fabrication method and optical circuit design principle, so that the branching ratio tends to vary according to input wavelengths and input ports.
For this reason, there arises the problem that the 2×2 coupler using the fusion coupler, or MZI-WINC has difficulty in holding the branching ratio constant for all use wavelengths and all input ports.
This 2×2 coupler also has difficulty in stable and low-cost fabrication because optical characteristics are easily varied by fabrication errors.
Further, there is the problem that although in the conventional waveguide type optical branching device using an asymmetrical X-branching optical circuit, or the optical branching device using an adiabatic coupler type optical circuit, asymmetrical branching ratios of e.g., 1:4 or more are verified to be effective, a waveguide type optical branching device with the branching ratio of 1:1 fabricated under actual fabrication conditions causes significant variations in the branching ratio dependent on input wavelengths and input ports, which cannot satisfy required performance of optical communication systems.
For that reason, in the waveguide type optical branching device 90 explained in FIGS. 12 and 13, in order to make the branching ratio constant regardless of input wavelengths and input ports, it is necessary to lengthen the coupling portion 95 (coupling length), or strengthen optical coupling in the coupling portion 95.
In the asymmetrical X-branching optical circuit and adiabatic coupler type optical circuit, according to an adiabatic theorem that describes wave propagation in a system in which core cross-sectional shape or structure varies in the light propagation direction, by lengthening the coupling length sufficiently, the branching ratio of the above optical circuit can be constant, but in practice, it is difficult to make the length of the optical circuit more than a certain value because of constraints in fabrication apparatus and fabrication cost. Also, in order to form the long coupling length, increasing the size of the waveguide type optical branching device is not desirable.
To realize an asymmetrical X-branching optical circuit with a constant branching ratio without lengthening the coupling portion 95, although it is necessary to narrow a gap G in the coupling portion 95 and thereby strengthen optical coupling between the two optical waveguides, because of the micron-sized gap G in the coupling portion 95, and therefore constraints in light exposure and etching techniques in conventional fabrication methods, it is difficult to make the gap G in the coupling portion 95 less than a constant value.
For the above reason, the conventional fabrication methods have difficulty in realizing an asymmetrical optical circuit with a constant branching ratio without increasing practical optical circuit size.